where \(\mathbf {\mathit {q}}\)  is water flux (m/s) and \(K = \rho g k/\mu \)  is the isotropic hydraulic conductivity (m/s). We can assume \(\nabla h\)  is vertically uniform because \(\psi =h\)  along any vertical line for a hydrostatic aquifer where there is no vertical flow. If \((K_{xy}/K_z)\nabla h \ll 1\) , where \(K_{xy}\)  is a representative horizontal conductivity and \(K_z\)  is the vertical conductivity, the approximation is reasonably accurate [1]}.
